Generally, items to be delivered by a postal network, such as the United States Post Office “USPS,” are affixed with an address which informs the postal network where the item should be delivered to. The address usually includes a geographical identifier (such as a ZIP code or postal code) which is often in the form of character and/or numerical strings. The geographical identifier identifies a specific geographical area, this area may be as large as several states or provinces, or smaller, such as a single office block or a number of residential properties, for example.
The geographical identifier tends to be of a hierarchical structure. For example, in the U.S, five-digit ZIP codes are often used to represent a defined area of destinations. The first digit represents a group of states (e.g. 6 represents Illinois, Kansas, Missouri and Nebraska), the second and third digits represent an area within that group of states (e.g. 06 represents Chicago). Following the hierarchical structure, the fourth and fifth digits represent a smaller area within the area represented by the preceding digits. An additional four digits are often utilized to define a more precise location. In the United Kingdom, postal codes are used as geographical identifiers. These comprise a one or two letter prefix that defines an area (for example a county), followed by a one or two digit number that defines a smaller area (for example a town), followed by a number and two letters that defines an even smaller area (for example eight houses).
An article to be delivered (“mail”) is typically posted into a mail box containing a number of other pieces of mail, each often having a different destination. These delivery articles are collected periodically and are taken to a mail processing facility which receives mail from many other mail boxes and other mail sources. Since all of the mail is not intended for the same address it must be sorted so that mail going to similar geographical places is grouped together. Often, a batch of mail is sorted by a mail sorting machine that sorts the mail based on the geographical identifier associated with each article. Mail sorting machines have a limited number of bins and as such it is not possible to provide a separate bin for each unique geographical identifier.
A sort plan defines how articles are to be sorted based upon their geographical identifier. In one example, mail having the same geographical identifier to the same predetermined hierarchical level is grouped together. For example, mail having a geographical identifier starting with ‘6’ may be sorted into the same bin (i.e. 6XXXX, where X represents any digit). This bin, or group, of mail needs to be further sorted since the geographical identifier prefix ‘6’ still refers to a relatively large area. Therefore, a further sort is performed on the group of mail. Mail having the same geographical identifier to a second predetermined hierarchical level may then be grouped together. For example, mail having a geographical identifier starting in ‘606’ may be sorted into the same bin (i.e. 606XX, where X represents any digit). Thus, one problem arises when determining how much mail should be classified together. The predetermined hierarchical level may vary depending on the volume of mail. Often, however, the volume of mail is not fixed and will fluctuate over time. Combining too much mail together may force additional costly sorts to further define specific groups within the original sort. Not combining enough mail may introduce inefficiency to the system. Further, because the number of sorts that takes place is proportionate to the cost of sorting the mail, it is desirable to reduce the number of sorts.
Currently, sort plans are fixed. Fixed sort plans are undesirable due to the fact that the volume of mail having a particular geographical identifier may vary hourly, daily, monthly and/or seasonally, for example. For example, on a weekday there may be a large volume of mail to be delivered to business addresses but on a weekend there may not be very much mail to be delivered to business addresses. However, even though the spread of mail is very different, the sort plan for sorting the mail is the same. Further, larger cities such as Chicago, New York, and Los Angeles traditionally have less mail during seasonal holidays as many residents travel to more rural areas to visit relatives and/or friends. Thus, for at least these reasons, fixed sort plans are not particularly efficient.